def find_diff(s, t):
    if not s:
        return t
    total = 0
    for i in range(len(t)):
        if i < len(s):
            total -= ord(s[i])
        total += ord(t[i])
    return chr(total)


s = 'ae'
t = 'eab'
r = find_diff(s, t)
print(r)
